In 2002, the National Cancer Institute identified the tumor microenvironment as a priority research area and initiated efforts to expand our knowledge of the cells and factors that normally populate the microenvironment, as well as to advance our understanding of how these microenvironment components interact with tumor cells. The Fifth International Conference on Tumor Microenvironment: Progression, Therapy and Prevention, is timely as many novel concepts have emerged in the field since that time and the contribution of the microenvironment to cancer progression is increasingly recognized. This conference is jointly sponsored by the International Cancer Microenvironment Society (ICMS), the American Association for Cancer Research (AACR), and the French Institut National du Cancer (INCa), and will be held in Versailles, France from October 20-24, 2009. The meeting will be held over 3.5 days and consist of 7 plenary sessions, 8 concurrent symposia, and 1 poster session. Plenary sessions will focus on Regulation of Gene Expression in the Tumor and in Non-Tumor Cells in the Microenvironment;The Role of the Microenvironment in Metastasis and Tumor Progression;Interactions of Tumor Cells with: Bone Marrow Cells, Immunocytes, Endothelium, Fibroblasts, ECM, and Soluble Immune Mediators (Antibodies, Cytokines, and Chemokines);Inflammation and Protective Immunity in the Tumor Microenvironment (two sessions);and Therapeutic Targeting of Tumor-Microenvironment Interactions: Pre Clinical and Clinical Studies (two sessions). The organizers'goals are to present a series of state-of-the-art lectures given by nationally and internationally renowned scientists from diverse backgrounds around a unified theme: "The microenvironment and its role in cancer progression, therapy, and prevention;" increase the awareness of the scientific community, and in particular more junior investigators and students, of the importance of this field of research in cancer and to encourage them to initiate investigations in this area;accelerate translational research by bringing together scientists, clinicians, academicians, and researchers of the industry;and provide ample opportunities for open discussion, scientific interaction, and collaboration among scientists with complementary expertise by conducting such a conference in a relaxed atmosphere. To hold another conference on this topic in 2009 is appropriate considering the rapid phase of development of this field of cancer investigation.